<!DOCTYPE html>
<html>
<head>
  <meta charset="UTF-8">
  <title>Document</title>
  <style>
    div {
      width: 200px;
      height: 200px;
      background-color: cadetblue;
      margin-bottom: 10px;
    }
    button {
      margin-left: 10px;
    }
    .change-border {
      border-radius: 50%;
    }
  </style>
</head>
<body>
  <div class="change d s"></div>
  <button class="btn">变成圆形</button>
  <button class="btn">变回方形</button>
  <script>
    var box = document.getElementsByClassName('change');
    var btns = document.getElementsByClassName('btn');
    btns[0].onclick = function () {
      box[0].classList.add('change-border')
    };
    btns[1].onclick = function () {
      box[0].classList.remove('change-border')
    };
  </script>
</body>
<html>